A study of the conditions of growth shows how such changes in the form of the body must develop. Setting aside the prenatal development, we find that at the time of birth some parts of the body are so fully developed that they are not far removed from their final size, while others are quite undeveloped. Thus the skull is, comparatively speaking, large at the time of birth, grows rapidly for a short time, but very soon approaches its full size, and then continues to grow very slowly. The limbs, on the other hand, grow rapidly for many years. Other organs do not begin their rapid development until much later in life. Thus it happens that retarding or accelerating influences acting upon the body at different periods of growth may have quite different results. After the head has nearly completed its growth, retarding influences may still influence the length of the limbs. The face, which grows rapidly for a longer period than the cranium, can be influenced later than the latter. In short, the influence of environment may be the more marked, the less developed the organ that is subject to it. Data on the unequal rate of growth of different parts of the body have been furnished by Weissenberg. The influence of retardation, so far as it has been studied, seems to be lasting. In other words, a retardation in development is never completely made good by long-continued development. When a child, through unfavorable influences, has grown slowly during a number of years, it will probably continue to grow longer than other, normal children; but the total amount of its growth will always remain too small (Boas and Wissler). On the other hand, children whose development has been accelerated will reach the adult stage early, but nevertheless the total amount of their growth will be relatively great. It follows from this consideration of the effect of retardation and of difference in period, that not only the absolute size, but also the relative proportions, of the body, must be influenced by periods of retardation or acceleration. The whole trend of the studies of growth thus emphasizes the importance of the effect of rate of development upon the final form of the body. Illness in early childhood, malnutrition, lack of fresh air and physical exercise, are so many retarding causes, which bring it about that the growing individual of a certain age is in its physiological development younger than the healthy, well-nourished individual, who has plenty of fresh air, and who puts his muscular system to good use. Retardation or acceleration has, however, the effect of modifying the later course of development; so that the final stage will be the more favorable, the less the retarding causes. It seems more than likely, judging from the course of development of a few simple mental activities that have been made the subject of study, that mental development follows laws quite analogous to those of physical development (Meumann).